1. Field of the Invention
The present invention relates to an optical head and, more particularly, it relates to an optical head incorporated into the optical information recording and reproducing apparatus for projecting laser beam on an optical medium to reproduce and record information from and on the optical medium.
2. Description of the Related Art
Various kinds of optical recording media such as the optical disk on which information can be optically recorded have been developed and as these optical recording media come into the market, the development of devices for reproducing information recorded on these optical recording media and for optically recording and reproducing information on and from these optical recording media has become more and more active and the improvement of optical heads which are to be incorporated into these devices has become more active as well. Further, various kinds of measures have been provided for positioning and adjusting optical parts which are important in the course of making these optical heads. The optical heads of this kind are adjusted according to the following two manners, for example.
FIG. 1 shows the optical system of the conventional optical head. In the case of this optical head, a divergent light beam emitted from a semiconductor laser 21 is collimated and converted to parallel beam by a collimator leans 22 and the parallel beam are focused on an optical recording medium 25 by an objective lens 4. Arranged between the objective lens 4 and the collimator lens 22 are a prism mirror 20 such as the triangular prism for reflecting the parallel light beam to the objective lens 4 and a beam splitter 23 for separating the detecting light beam reflected by the optical recording medium 25 from the light beam emitted from the semiconductor laser 21 to the optical recording medium. The detection light beam from the beam splitter 23 is lead to an optical detector 24 through a optical detecting system 24a.
In the case of the optical system shown in FIG. 1, the prism mirror 20 is arranged as shown in FIG. 2. The mirror 20 is fixed on a mirror mount 26, which is fixed to a base 27 by means of a fixing screw 29. Arranged around the fixing screw 29 are three adjusting screws 28 for adjusting the tilt of the mirror mount 26. The optical system of this optical head is adjusted in such a way that relative positions of the semiconductor laser 21 and the collimator lens 22 are adjusted at first in directions X, Y and Z shown by the three-dimensional coordinates and that the parallel degree and travelling direction of the light beam are then adjusted after the light beam passes through the collimator lens 22. The beam splitter 23 is kept fixed over this process. The adjusting screws 28 shown in FIG. 2 are then turned and the tilt of the mirror 20 is thus adjusted to make the optical axis H of the mirror 20 substantially perpendicular to the optical recording medium 25. Further, the objective lens 4 is moved in the directions X and Y to cause its optical axis to coincide with the optical axis H and then turn-adjusted in directions shown by arrows o and Y in such a way that the aberration of beam spot formed on the optical recording medium becomes the smallest.
After the adjustment of the beam emitting and focusing optical system is finished as described above, the beam detecting optical system 24a and the optical detector 24 are adjusted and the adjustment of the optical head is thus finished.
According to the adjusting method of the optical head which comprises adjusting the tilt of the prism mirror 20 to cause the optical axes of the optical systems to coincide with each other as described above, however, the optical axis H of the mirror 20 which advances to the objective 4 is tilted at the time when the mirror 20 is adjusted. In order to cause the optical axis H of the mirror 20 to coincide with that of the objective 4, therefore, the mirror 20 must be adjusted in many directions such as X, Y, Z, .alpha. and .gamma., and the number of steps for adjusting the mirror 20 is large, too. Further, when the optical axis H of the mirror 20 is tilted, the position of readable end point on the optical recording medium is shifted relative to the optical head.
FIG. 3 shows an optical head of separation type capable of being made small in size and light in weight, enhancing its productivity, being driven at high speed without causing any vibration and any tilting of its optical axis, and realizing high speed recording and reproducing of information. In the case of this optical head, the objective lens 4 for converging optical beam on the optical recording medium is held and driven in the direction of optical axis by a focus actuator 1. The focus actuator 1 is made of resin and molded integral to a linear motor 50 which is also made of resin and which serves to drive the objective 4 in a direction perpendicular to the direction of optical axis. A center carriage 11a for enhancing the rigidity of a tracking coil 9 is made cylindrical and provided therein with a hollow portion 11h through which optical beam can be passed. As shown in FIGS. 4 and 5, a prism mirror 31 is bonded to the carriage 11a under the objective 4 and the fixed beam sending optical system including a light source and a collimator lens, the fixed optical system for detecting light beam, and the optical recording medium or optical disk ar optically connected through the prism mirror 31. The beam sending optical system for transmitting light beam to the optical recording medium 25, the beam detecting optical system for detecting optical beam reflected by the optical recording medium and a detector may be mounted on the optical head.
In the case of the optical head arranged as described above, the tilt of the prism mirror is adjusted as follows to cause its optical axis to coincide with that of the objective.
Adhesive 30 is coated on a mirror-bonded area 11i extending from the bottom of the hollow portion 11h of the center carriage 11a. The prism mirror 31 which is shaped like a triangular pole is mounted on the layer of adhesive 30 and the tilt of the prism mirror 31 is adjusted until the adhesive 30 is hardened.
The reason why the adhesive layer is interposed between the prism mirror and the mirror-bonded area is as follows.
When the mirror 31 is bonded and fixed directly on the mirror-bonded area 11i without adjusting its tilt, it is difficult to completely align the optical axis of the prism mirror 31 with that of the objective lens 4 because the assembling accuracy of the linear motor 50 and the accuracy of parts for the linear motor 50 cannot be kept certain. When the adhesive 30 is interposed between the prism mirror 31 and the mirror-bonded are 11i and the tilt of the prism mirror 3 is adjusted in a certain direction prior to the hardening of the adhesive 30, the tilting of the prism mirror 31 can be easily achieved and the stable optical characteristic thereof can be realized. When the prism mirror 31 is previously adjusted to have a certain position and a certain tilt as described above, it can be chucked and held at its face except its reflecting and bonded faces 31a and 31b at the time of its being bonded on the mirror-bonded area 11i.
As described above, the prism mirror 31 is located relative to the mirror-bonded area 11i which is formed integral to the carriage 11a with a certain clearance interposed between them, and this clearance between them is filled with the adhesive 30. This is because the tilt of the optical axis of the prism mirror 31 can be easily adjusted when the adhesive 30 is present in the clearance and the tilt of the prism mirror 31 is adjusted in a certain direction prior to the hardening of the adhesive 30.
According to this manner, the prism mirror 31 must be held at a certain position in order that the prism mirror 31 is previously adjusted to occupy a certain position and have a certain tilt. Therefore, the prism mirror 31 is directly held with strong force but when it is held with strong force, its mirror face is deformed. In order to solve this, that face of the prism mirror 31 which is in opposite to its reflecting face 31a is used as a chucking face which is sucked by the vacuum pump or chucked by the adhesive tape to hold it at the time when it is bonded on the mirror-bonded area 11i. When it is held in this manner, however, it cannot be avoided that its chucking becomes unstable.
Further, the prism mirror 31 is bonded on the mirror-bonded area 11i over its bonded face by the adhesive 30. When adhesive, low in hardness, is used, therefore, unnecessary vibration such as tilting vibration is caused and the mirror face of the prism mirror 31 is deformed by the hardening of the adhesive and by the thermal deformation of it after it is hardened.